1. Field of the Invention
This invention generally relates to a developing device for developing an electrostatic latent image formed on an image bearing member to convert the latent image into a visual image, and in particular, to a developer transporting unit for transporting as carried thereon a layer of developer to be applied to the latent image for visualization. More specifically, the present invention relates to a floating electrode type developing device which includes a plurality of small-sized electrodes which are electrically floating and function as auxiliary electrodes.
2. Description of the Prior Art
A device for developing an electrostatic latent image formed on an image bearing member comprised of a conductive support and a photoconductive layer formed on the support has been widely used in electrophotographic copiers, electrostatic recording machines and various other types of machines. In the developing technology, electrostatic latent images may be categorized into two different classes depending upon the degree of their spatial frequencies. One of them includes "line images" which are mainly comprised of higher spatial frequency components, and the other class includes "area images" having lower spatial frequency components. The line image is an image mainly formed by lines appropriately arranged to indicate a pattern or character; on the other hand, the area image implies an image having a relatively large two-dimensional section to be developed such as a picture.
It is to be noted that required developing conditions differ depending upon the class of image, i.e., whether it is a line image or area image. Stated more in detail, in the case of area images, it is normally required that the developing density vary depending upon the level of the surface potential of an electrostatic latent image to be developed so as to express tone or shading variation. On the other hand, in the case of line images, the developing density is normally required to be always high irrespective of the level of the surface potential of a latent image to be developed. In other words, a line image is usually desired to be developed to a high density image even if the surface potential of its latent image is very low.
Such dual requirements in development are satisfied as long as use is made of a two component developer comprised of toner and carrier beads. However, in developing devices using a single component developer comprised of magnetic toner particles, difficulty has been experienced in satisfying the above-noted dual requirements. Under the circumstances, there has also been proposed an improved developing device using a single component developer which could satisfy the above-noted requirements as disclosed in the Japanese Patent Application No. 55-185726. The proposed developing device is characterized by using a novel developer carrier including a conductive support and a plurality of conductive particles provided on the support such that the particles are electrically isolated from one another as well as from the support thereby the conductive particles function as fine floating electrodes. With such a structure, the above-noted dual requirements may be satisfied sufficiently.
Several methods for manufacturing a developer transporting unit having the above-described structure have also been proposed as will be briefly reviewed below.
(a) Conductive particles of metal are first mixed with a dielectric resin material and such a mixture is then deposited on a conductive support.
(b) An adhesive agent is first applied to a conductive support and then conductive particles are spread thereon.
(c) A dielectric layer is first formed on a conductive support and then a conductive layer is formed on the dielectric layer. Then the conductive layer is converted into a pattern of islands each forming a fine electrode, for example, by etching.
However, in accordance with the above-described method (a), as the mixture ratio of the particles increases as compared with the resin, it becomes increasingly difficult to form a layer of the mixture on the support. Thus, the mixture ratio is rather limited. The above-described method (b) also suffers from disadvantages because the resulting surface is significantly irregular and thus it is impossible to form a developer layer of uniform thickness. Besides, the particles are rather prone to come off, indicating a poor service life. The prior art method (c) tends to be expensive thereby necessarily increasing manufacturing cost.